Self-routing packet switching networks such as those using banyan switching nodes communicate packets on the basis of address information contained within the packets. One such switching network is disclosed in the application of J. S. Turner, Case 8, Ser. No. 393,112, which was filed on June 25, 1982, and allowed on July 23, 1984, now U.S. Pat. No. 4,512,011, and assigned to same assignee as this application, and is illustrated in FIG. 1. In the latter figure, there is only one unique route between each input and output pair of the network. For example, there is only one path between trunk controller 100-0 and trunk controller 112-0. This path is via nodes 102-0, 104-0, 106-0, 108-0, 110-0 and links 101-0, 103-0, 105-0, 107-0, 109-0, and 111-1. Node 102-0 is responsive to address information within the packet to route the packet to node 104-0, and subsequent nodes in the previously described path are responsive to the address information to properly route the packet until it is received by trunk controller 112-0. In addition, trunk controller 100-0 shares a portion of this path to trunk controller 112-0 with trunk controller 100-2 since trunk controller 100-2 only has one path to trunk controller 112-0, and this path is via nodes 102-2, 104-2, 106-2, 108-0, and 110-0. These two paths initially meet at node 108-0 which can have a maximum of 16 trunk controllers attempting to transmit information to one of four trunk controllers attached to nodes 110-0 or 110-1. When such an unbalanced traffic condition occurs, the traffic capacity is limited to the maximum traffic capacity of node 108-0. It is important to realize that the traffic concentration can be even higher on node 106-0 than previously described. In addition, to the problems of unbalanced traffic, if node 108-0 fails, then a large number of trunk controllers cannot communicate with certain other trunk controllers.
One known method for alleviating the reliability and traffic problems in a self-routing network is discussed in the report entitled, "Development of a Voice Funnel System", Bolt, Beranek and Newman, Inc., Report No. 4098, August, 1979, pages III-29 through III-76, which discloses the use of an extra stage of banyan switching nodes at the input of a self-routing network in an attempt to resolve the previously mentioned problems. The report proposes that this extra stage of switching be identical to other stages of the network and be utilized by adding an extra bit of addressing to the address field of each packet being routed through the switching network. This extra stage of switching would precede stage 1 of the shown in FIG. 1. The extra address bit would be controlled by hardware or software external to the switching network and would determine the route through the switching network. The hardware or software would use this bit so as to avoid a node which was failing or experiencing heavy traffic.